1. Field of the Invention
The present invention relates to a wireless communication apparatus for performing wireless communication using an antenna selected from a plurality of antennas, and a control method for the wireless communication apparatus.
2. Description of the Related Art
In recent years, an electronic apparatus having a wireless communication function for a wireless LAN, Bluetooth®, or the like is spreading. The wireless LAN, Bluetooth, or the like uses a 2.5 or 5 GHz-band radio wave. The above electronic apparatus having a wireless communication function incorporates, in its housing, an antenna for wireless communication for which various antennas such as a dipole antenna, helical antenna, slot antenna, and inverted F antenna are used.
If an electronic apparatus such as a personal computer incorporates an antenna, a body may move close to the antenna. In this case, the characteristics of the antenna change to cause deterioration of the performance, and the body absorbs a radio wave emitted by the antenna. In consideration of the influence of a radio wave on a body, the standard for an SAR (Specific Absorption Rate) representing the degree of energy of a radio wave absorbed by a body is stipulated. In view of such a situation, it is required to reduce the amount of radiation of a radio wave to a body while maintaining a communication state.
FIG. 3 is a graph in which the ordinate represents a change in received power of a reception unit and the abscissa represents a communication distance. FIG. 3 shows a case in which the received power decreases as the communication distance is longer. A conventional antenna diversity scheme maintains the communication quality by switching an antenna in use to another antenna when the received power of a communication partner becomes smaller than a threshold Pth. It is a common practice not to switch the antenna when the received power is equal to or larger than the threshold Pth, because the received power level is sufficient for communication.
There is also well known a control operation of avoiding deterioration of the characteristics of the antenna due to its surrounding environment. A technique of appropriately controlling the impedance of an antenna when a body moves close to the antenna in order to avoid the influence of the body is known as Japanese Patent Laid-Open No. 2005-354502 (to be referred to as patent literature 1 hereinafter). Patent literature 1 discloses a technique in which in a wireless communication apparatus, such as a cellular phone, for transmitting/receiving data, an impedance mismatch occurring when a body moves close to the apparatus is solved to reduce a power loss due to the impedance mismatch. An adaptative control unit measures the detected value of reflected power, and reads out a phase angle and capacitance value from a storage unit, thereby adaptively controlling the phase angle and a variable capacitance capacitor based on a measurement result so that the reflected power becomes smallest.
The conventional antenna diversity scheme of ensuring the communication quality by switching an antenna does not reduce the influence of a body or an influence on the body. Furthermore, a control circuit in an antenna circuit, which solves an impedance mismatch in order to avoid the influence of the body is complicated, and the scale of the circuit around the antenna is large.